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Bacteria: A Secret of Breastfeeding? Features
Dr Steven Lin
Photo: Jen Barber & Daisy by Chris Davison

The microbial population living in breast milk and the transport of bacteria between mother and child play a crucial role in regulating health.

With no more intimate moment between a mother and child, breastfeeding provides nutritional, immunological and emotional nurturing. However, recent discoveries have revealed an even deeper connection that suggests breastfeeding is far more precious than we first thought. The secret lies within microbial populations living in breast milk. Whilst bacteria hardly sound like an enticing gift, the transfer of a mother’s microbial inhabitants appear to be the most valuable gift a mother can provide to her child.


Definitely not sterile

Not so long ago, it was thought that breast milk was completely sterile. Any sign of bacteria was attributed to the presence of infection in the breast, such as mastitis, or to contamination by skin flora. However, advances in understanding of the vast microbial populations that live within our body, dubbed the ‘human microbiome’, paint a completely different picture.

Healthy breast milk has been found to be crawling with bacteria. DNA sequencing technology is now identifying 400 species living in the breast with many more likely to be accompanying them. To make things even more complicated, they are constantly changing over time with measured shifts in microbial presence between pre- and postnatal samples, as well as between months 1-6 of breastfeeding.


A mother’s inner highway

For a newborn, the first encounter with bacteria occurs with the placenta. Then, natural vaginal birth is designed to provide a ‘starter pack’ of microbes for the child.

The mother’s body doesn’t stop there: it will prepare a rich and diverse microbial population to be delivered during breastfeeding. Bacteria, as it turns out, seem to be traveling from the mother’s gut to her breast milk. New and exciting research is showing how the immune system moderates this process in the stomach by selecting microbes to transport to the breast via the lymphatic system.

Like a microscopic highway, studies have also shown that breastfeeding women given a capsule of bacteria to swallow can later have the very same species isolated from their breast milk. A fascinating picture of how a mother cherry picks microbes from the outside world to give to her infant.


Your child’s lifelong friend

So why does the human body have such a well designed system to pass on its tiny inhabitants? Microbes are now known to outnumber human cells by 10 to 1. Our own body is more bacteria than ‘human’ and the role of bacteria in controlling body processes has only recently been discovered.

Microbes play a significant role in a newborn’s digestion of breast milk. They also join with the infant’s immune system to act as a ‘bodyguard’ that keep harmful foreigners under control. Initially, in order for friendly microbes to enter, the infant’s own immune system temporarily shuts down, to allow the colonisation of the mouth and gut.

During this period, the microbial population first occupies the oral cavity. Over the next two weeks the mouth then ‘seeds’ the child’s gut population. Once inside the stomach, microbes team up with gut and immune cells to form the sealed barrier known as the ‘gut lining’. This is the child’s primary immunological interface with the world, as well as a mediator for metabolic, brain, and digestive functions. Later in life, a loss of diversity of microbes in the mouth has been shown to relate to diseases like tooth decay, whilst in the stomach it can result in immunological, digestive and many other diseases.


Bacteria: a secret of breastfeeding?

Breastfeeding and the deep emotional connection it creates seem to coincide with the transport of bacteria between mother and newborn. Whilst we’ve placed much focus on preventing the exposure of children to bacteria, a new dawn of knowledge is revealing that microbes play a crucial role in our body.

Key to the transport of bacteria from mother to newborn is a natural, vaginal birth and breastfeeding for at least six months. Researchers also need to learn more about the mother’s own microbiome, which determines her ability to pass on a healthy balanced ecosystem to her child.


Fernández L, et al. The human milk microbiota: Origin and potential roles in health and disease. Pharmacol Res, 2012, ar; 69(1):1-10. doi: 10.1016

Jeurink, P, Van Bergenhenegouwen, J.,  Jiménez, E. et al. Beneficial Microbes, March 2013; 4(1): 17-30

Rogier, E. 2014. Secretory antibodies in breast milk promote long-term intestinal homeostasis by regulating the gut microbiota and host gene expression. PNAS; 3074–3079, doi: 10.1073/

Jost, T., et al. 2014, Vertical mother–neonate transfer of maternal gut bacteria via breastfeeding. Environmental Microbiology, 16: 2891–2904. doi: 10.1111/1462-2920.12238

 Zhang, A. et al. 2012. Human Whey Promotes Sessile Bacterial Growth, Whereas Alternative Sources Of Infant Nutrition Promote Planktonic Growth, Current Nutrition and Food Science, ISSN: 2212-3881

Dr Steven Lin



Dr Steven Lin is a dentist, writer, and TEDx Speaker. He was trained at the University of Sydney, with a background in biomedical science, nutrition and public health education. Currently he is working on his publication, The Dental Diet: an exploration of evolutionary diet, genetics and nutritional medicine.




  1. […] Bacteria: A Secret of Breastfeeding? […]

  2. Great article. Would be interested to know about the health benefit of providing breast milk to your toddler. It is it possible to rebuild or reprogram a child immune system? My 3 years old have Alopecia the last few months. Within 3 months, she completely lost all her hair. I been giving her my breast milk, hoping that it will help reprogram her immune system. Hoping that some how make her immune system realize that her hair follicle cells are not foreign. I am interested in finding for information about the immune system and research out there about it. I hope there are more research done for Alopecia in children, and find out the causes and cure it. It’s really tough seeing my daughter without hair.

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